When the operating system instructed one of the active cores to enter C3 sleep state using the Advanced Configuration and Power Interface (ACPI), the other active core(s) dynamically accelerated to a higher frequency.

I realized that you need to make sure you have the ACPI C3 state option enabled in your BIOS. Some users who are looking to make their i7 perform the highest by disabling power saving features may actually be disabling the highest speeds for single core operations. I enabled this and now I get 3.9Ghz speeds:

Moral of the story – don’t disable ACPI to try to gain faster speeds, you might lose single/duel core turboboost!